The invention relates to a heat-exchanger tube with optionally smooth ends, at least one structured section on the outside and inside of the tube and optionally smooth intermediate sections, whereby the outside diameter of the structured area is no greater than the outside diameter of the smooth ends or of the smooth intermediate areas.
This type of tube is usually identified as xe2x80x9cdouble-enhanced tubesxe2x80x9d.
Heat-exchanger tubes of the mentioned type are usually used in shell and tube heat-exchangers (see FIG. 1, Source: TEMA, Standards of Tubular Exchanger Manufacturers Association, New York, 1968). These heat exchangers are characterized by a plurality of tubes 30, which are arranged parallel to one another, and which at their ends are tightly connected to the tube sheets 31. Depending on the operating conditions and tube length, the tubes are supported by means of baffles 32. These baffles 32 are also utilized to direct the shellside fluid flow in specific directions. For example, water or a mixture of water and glycol flows in the tubes 30, whereby the flowing medium along the inside of the tubes is heated or cooled off.
In order to increase the heat-transfer performance of such heat exchangers, finned or structured tubes instead of smooth surfaced ones are utilized. It is hereby intended to enlarge the surface which is available for the heat transfer and to furthermore utilize effects of the surface tension. FIG. 2 illustrates schematically a structured heat-exchanger tube 30. It has several structured sections 2, which are confined by smooth, unstructured end sections 1a and smooth unstructured intermediate sections 1b .The tube 30 is usually tightly connected at the smooth end sections 1a to the tube sheets 31 through a rolling process. The tube 30 rests at the smooth intermediate sections 1b in the bores of the baffles 32. In order for the tube to be able to be moved into the tube sheets 31 and baffles 32 and to be able to be tightly connected to the tube sheets 31 or not to have too much clearance in the bores of the baffles 32, the outer diameter of the structured sections 2 may not be greater than the outer diameter of the smooth sections 1a and 1b .On the other hand, the inside diameter of the tube 30 should be as large as possible in the structured sections 2 in order to keep the pressure drop of the tubeside flowing medium as low as possible. The outside and inside diameter of the tube 30 are at a given structure type in relation to one another in the structured section 2 so that also the outside diameter of the tube 30 should be as large as possible in the structured section 2. Thus, it is advantageous to choose the outside diameter in the structured section 2 to be almost equal to the outside diameter of the smooth tube sections 1a and 1b. 
In order to lower the material costs of such tubes, the specific tube weight (i.e. tube weight per unit of length) of the tubes must be reduced at a specified tube diameter. Since the minimum wall thickness is limited by safety requirements, a reduction in the specific tube weight can only be achieved through a reduction of the weight of the structure. An increase of the heat-transfer surface through structuring with a simultaneous minimizing of the structure weight requires a very fine, slim structure.
The use of double enhanced tubes is state of the art in some parts of the industry (for example, in chillers for air conditioners). Many of these tubes are based on finned tubes, whereby the fin tips were modified through notching and flattening. Such tubes are usually manufactured using a rolling process: Rolling disks with a specific profile shape are set up with an increasing diameter on one or more tool shafts. These tool shafts are arranged evenly around the periphery of the tube to be worked. When the inclined positioned, rotating tool shafts are moved towards the smooth tube, the rotating rolling disks penetrate into the wall of the tube, rotate the tube, advance it corresponding to their inclined position in axial direction to form radially outwardly extending helical fins out of the wall of the tube. This operation is similar to a thread rolling operation. Examples for this technology are illustrated in U.S. Pat. Nos. 2,868,046, 3,327,512, 3,383,893 and 3,481,394.
The tube is during the rolling process supported by a mandrel lying in the tube, which mandrel absorbs the radial forces. In order to produce an inner structure, profiled mandrels are provided with helical grooves (DE 23 03 172 C2). Since the inner structure of the tube is determined by the profile shape of the mandrel, it can be shaped essentially independent from the geometry of the outer fin structure. Thus, it is possible to optimally adapt the outer and inner structure independent from one another to accommodate various operating conditions. The mandrel must rotate at a certain speed in order to unscrew itself from the inner structure of the tube. This produces high friction forces between the mandrel and the tube, which must be applied by the rolling disks in order to cause the advance of the tube in the axial direction. A considerable portion of these friction forces is directed parallel with respect to the tube axis 33 and thus also almost parallel with respect to the axis of the rolling disks.
It is known that it is advantageous for certain applications (for example, refrigerant evaporators and condensers) to use structures with small fin pitches in order to achieve an increase in the heat-transfer performance. In the past, fin pitches of 1.35 mm (19 fins per inch) have been used. Today finned tubes having fin pitches of approximately 0.40 mm are commercially available (U.S. Pat. No. 5,697,430 and DE-19757 526). EP-0 701 100 A1 shows that the trend is going to yet smaller pitches (0.25 mm).
Smaller fin pitches demand thinner rolling disks, which causes an increased danger regarding breakage due to the above mentioned friction forces and a greater susceptibility to wear of the tool. The tool life thus becomes more critical and repeated production interruptions because of tool exchange are the consequence. Furthermore the production speed of the rolling machines decreases with decreasing fin pitch. At the same time, because of worldwide competition, the production costs become a decisive factor for the economical success of the manufacture of structured tubes.
Therefore the basic purpose of the invention is to manufacture a delicately structured tube which has both on the outside and also on the inside a large increase in surface area and has a low structure weight. The geometries of outer and inner structures are adaptable independent from one another. The tube must be able to be manufactured at a high speed, with simple tools and low tool wear. Smooth end sections and intermediate sections are manufactured without extra expense.
The purpose is attained according to the invention by creating recesses with certain dimensions on the outside and ribs with certain dimensions on the inside of the tube. The recesses and ribs are formed by pressing rotating roll-forming tools into the tube wall and by the displaced material of the tube wall being pressed inwardly onto a profile mandrel lying in the tube. The utilized structuring tools can be adjusted in such a manner that they create both aligned, continuous grooves and also non-aligned, spaced-apart recesses.
By using additional tools, it is possible to modify the recesses so that secondary structures are created at the flanks or at the base of the recesses or at the ribs between the recesses. Depending on the use, these secondary structures can significantly increase the thermal performance of tubes. This occurs essentially by utilizing surface-tension effects.
It is advantageous for condenser tubes to create structures which have convex edges and channels extending essentially in peripheral direction. These channels enable the discharge of condensate, which is generated on the tube itself or on the tubes of the tube bundle being arranged thereabove.
It is advantageous for tubes, which are utilized in flooded evaporators or spray evaporators, to produce structures with cavities by partially closing off the upper areas of the recesses. This is achieved according to the invention by additional flattening tools which are arranged downstream of the primary structuring tool on the tool shaft.